The present invention relates to a polymer electrolyte fuel cell stack that works at ordinary temperature and is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems.
The polymer electrolyte fuel cell generates electricity and heat simultaneously by reacting a fuel, such as hydrogen, and an oxidant gas, such as air, electrochemically at gas diffusion electrodes with a catalyst like platinum carried thereon.
One example of the polymer electrolyte fuel cell stack is shown in the partially omitted perspective view of FIG. 4.
On the opposite faces of a polymer electrolyte membrane 3, which selectively transports hydrogen ions, catalytic reaction layers 2, which are comprised of carbon powder with a platinum metal catalyst carried thereon, are closely formed. Additionally, if required, a fluorocarbon water repellent may be added.
The polymer electrolyte used here may be a fluorocarbon polymer with sulfonate groups introduced on the ends of their side chains. This electrolyte has proton conductivity in the wet state. In order to activate the fuel cell, it is accordingly required to keep the polymer electrolyte in the wet state. The polymer electrolyte in the wet state has strong acidity due to H+ dissociated from the sulfonate groups at the ends. Accordingly, sufficient acid resistance is required for materials that are in direct contact with the electrolyte. The materials equivalent to the electrolyte materials are also admixed to the reaction electrodes, so that acid resistance is also required for materials that are in direct contact with the reaction electrodes.
Further, on the respective outer faces of the catalytic reaction layers 2A, a pair of diffusion layers 1 having both gas permeability and electrical conductivity are closely formed. This catalytic reaction layer 2 and the diffusion layer 1 constitute an electrode (either an anode or a cathode).
In the case where pure hydrogen is used as the fuel, the same material can be used as the material for the anode and cathode. In the case where the fuel is a gas mainly containing hydrogen, which is obtained by reforming a hydrocarbon fuel, carbon monoxide is naturally contained in the reformed gas. In order to prevent the noble metal catalyst from being poisoned with carbon monoxide, adding an anti-CO poisoning substance, such as ruthenium, to the anode side only has been proposed.
Outside of the electrode, conductive separator (bi-polar) plates 4 are further arranged so as to mechanically fix the assembly of these electrolyte membrane and the electrodes and connect adjoining assemblies electrically with each other in series. In a portion of the separator plate 4 that is in contact with the electrode, a gas flow path 5 is formed to feed the supply of the reaction gas to the surface of the electrode and to allow the gas evolved by the reaction and the remaining excess gas to flow out. Gas manifolds 8 supply a gas to and exhaust gas from the fuel cells, and water manifolds 14 supply water for cooling the fuel cell stack down and also exhaust the water. A cooling means such as a cooling plate may also be provided to the separator plate 4.
In order to prevent hydrogen gas and air from leaking from the cell laminate or from being undesirably mixed with each other, an internal sealing structure is generally employed, in which sealing portions or O-rings are disposed around the electrodes across the polymer electrolyte membrane.
Since the above-mentioned proton-conductive electrolyte has strong acidity, a fluorocarbon polymer material having high acid resistance is employed for gasket-like sealing portions that are in direct contact with the electrolyte.
With a view to maximizing the area of the electrodes, an external sealing structure, which does not use the sealing portions or O-rings around the electrodes but extends the ends of the electrodes to the side face of the cell laminate and seal the side face of the cell laminate with an air-tight non-conductive material may be adopted.
Polymer electrolyte fuel cell stacks with an external sealing structure are divided into an internal manifold type and an external manifold type. In the internal manifold type, the manifolds or gas flow paths for feeding a supply of gas to the respective unit cells are formed inside the cell laminate in the form of through apertures that pass through the constituents of the cell laminate such as separators. In the external manifold type, on the other hand, the manifolds are arranged outside the cell laminate.
In one prior art method, a solution obtained by dissolving a resin in a solvent is applied and dried, or a reactive resin is applied and solidified, in order to form the gas seal portion that covers the side face of the cell laminate. However, sufficient gas sealing may not be obtained.
When the manifolds which connect with gas inlets and outlets are provided, the significant unevenness on the surface of the gas seal formed by the resin makes it difficult to ensure favorable gas sealing at a portion where the side face of the cell laminate is in contact with the manifold.
One method of sealing involves casting a thermosetting resin such as an epoxy resin into a cast mold which envelopes the cell laminate to integrally mold it, but solidification of the resin is time-consuming resulting in poor manufacturing productivity.
The above methods have another problem in that the gas inlets and outlets may be closed by the air-tight non-conductive material.
Around the electrodes, gasket-like sealing portions like gaskets are disposed and sandwiched between a pair of separator plates in order to prevent the reaction gases fed to the cathode and the anode from being leaked. One prior art technique arranges hard gaskets composed of, for example, a fluorocarbon resin, around the peripheral portion of the electrodes and subsequently places a pair of separator plates across the gaskets, but this requires accurate adjustment of the thickness of the electrodes and the gaskets.
When the gaskets have rubber-like elasticity, however, strict size accuracy is not required, though better functioning of the gaskets can be obtained by a certain level of adjustment of the thickness. Accordingly, the gaskets should have suitable acid resistance and rubber-like elasticity. Although having poorer acid resistance than fluorocarbon resin, ethylene-propylene-diene rubber (EPDM), which has suitable elasticity, is sometimes used for the material of the gaskets.
The separator plates are directly in contact with the electrodes and are thus required to have high gas tightness and electrical conductivity, as well as acid resistance. When air is used as the oxidant gas, it is necessary to enhance the flow rate of the air supplied to the cathode and to efficiently remove liquid water or water vapor produced at the cathode. A complicated structure generally called a serpentine-type structure, as shown in FIG. 5, is typically used for the gas flow path structure in the separator plate. The separator plate is obtained by cutting a carbon material such as a dense carbon plate having gas tightness, a carbon plate impregnated with a resin, or glassy carbon to a desired shape and forming grooves for gas flow paths. In another example, the separator may be obtained by processing and plating a corrosion-resistant alloy plate with a noble metal.
Also, the carbon material or the corrosion-resistant metal material may be used only for the portions that are in contact with the electrodes and require sufficient electrical conductivity. It has been proposed that separator plates of a resin-containing composite material be used for peripheral portions, such as manifolds, which do not require electrical conductivity. Also, it has been suggested that a resin be mixed with carbon powder or metal power and press-molded or injection-molded and used for the separator plates.
However, the fluorocarbon material employed for the gasket-like sealing portions is expensive. Additionally, the fluorocarbon material is generally a very hard resin and requires an extremely large load to clamp the gaskets and sufficiently seal the flow of gas or cooling water. Therefore, there have been attempts to use porous fluorocarbon material or to apply fluorocarbon paste on the separator plates, which are used in dry or half dry state. However, the porous fluorocarbon material is expensive. Additionally, when a load is for clamping which gives sufficient sealing properties is applied, this load may damage the porous fluorocarbon material.
Fluorocarbon paste used for the sealing portions is also high in material cost. Further, when dried and cured, its hardness makes it difficult to regulate its thickness at the time of the application. A rubber material like EPDM does not have as a high acid resistance as the fluorocarbon resin and is thus not suitable for long-term use. Additionally, EPDM generally exhibits thermoplasticity such that it is deformed over time at typical cell-driving temperatures, generally about 80° C. In some cases, the deformation blocks the gas flow path and lowers the supply of the fuel.
With respect to the material for the separator plates, in the case where dense carbon plate having gas tightness or glassy carbon is employed for the separator plates, cutting work is required to form the gas flow paths. This negatively affects mass production and manufacturing cost. In the case where carbon plate impregnated with a resin is used for the separator plates, impregnation of the resin after formation of gas flow paths causes warping of the carbon plate because of the low elasticity of the resin. Post treatment, including cutting the gas flow paths, should accordingly occur after impregnation of the resin. When a phenol resin or a silicone resin is used as the impregnating material, sufficient acid resistance cannot be obtained. In the case where a corrosion-resistant alloy or material plated with a noble metal is used, cutting work is required to form the serpentine flow path structure.
In the case where the mixture of a resin and carbon powder or metal powder is press-molded or injection-molded into separator plates, the resin itself is required to have acid resistance. Fluorocarbon resin or other hard resin materials have low fluidity and are difficult to mold. Because the resin has poor fluidity, only a small amount of the resin is mixed in the mixture. In this case, post treatment, for example, impregnating the portions that require the gas tightness with the resin, is required after molding. This results in a complicated structure.
The object of the present invention is thus to provide a polymer electrolyte fuel cell stack having seals of excellent durability. Another object of the present invention is to provide a method of manufacturing such a polymer electrolyte fuel cell stack with a high productivity.